Precursors for deposition of metal oxide layers or films

ABSTRACT

Rare earth metal precursors, for use in MOCVD techniques have a ligand of the general formula OCR 1 (R 2 )CH 2 X, wherein R 1  is H or an alkyl group, R 2  is an optionally substituted alkyl group and X is selected from OR and NR 2 , wherein R is an alkyl group or a substituted alkyl group. Methods of making such precursors and methods of depositing metal oxide layers from such precursors are also described.

This invention concerns precursors for deposition of metal oxide layers or films, methods of making such precursors and methods of depositing metal oxide layers or films using such precursors. This invention is particularly, but not exclusively, concerned with precursors for the growth of praseodymium oxide and other lanthanide (rare earth) metal oxides by chemical vapour deposition.

Rare-earth oxides M₂O₃ (M=Pr, La, Gd, Nd) are good insulators due to their large band-gaps (eg. 3.9 eV for Pr₂O₃, 5.6 eV for Gd₂O₃), they have high dielectric constants (Gd₂O₃ κ=16, La₂O₃ κ=27, Pr₂O₃ κ=26-30) and higher thermodynamic stability on silicon than ZrO₂ and HfO₂, making them very attractive materials for high-κ dielectric applications. Another attractive feature of some rare earth oxides (eg. Pr₂O₃, Gd₂O₃) is their relatively close lattice match to silicon, offering the possibility of epitaxial growth, eliminating problems related to grain boundaries in polycrystalline films.

Metalorganic chemical vapour deposition (MOCVD) is an attractive technique for the deposition of these materials, offering the potential for large area growth, good composition control and film uniformity, and excellent conformal step coverage at device dimensions less than 2 μm, which is particularly important in microelectronics applications.

An essential requirement for a successful MOCVD process is the availability of precursors with the appropriate physical properties for vapour phase transport and a suitable reactivity for deposition. There must be an adequate temperature window between evaporation and decomposition, and for most electronics applications oxide deposition is restricted to temperatures in the region of 500° C., to prevent degradation of the underlying silicon circuitry and metal interconnects.

Pr₂O₃ thin films have previously been deposited by physical vapour deposition techniques such as MBE and pulsed laser deposition. Metalorganic chemical vapour deposition (MOCVD) has a number of potential advantages over these techniques, such as large area growth capability, good composition control, high film densities and excellent conformal step coverage, but there have been very few reports on the MOCVD of Praseodymium oxide, due largely to a lack of suitable precursors.

Recently the MOCVD of a range of praseodymium-oxides (PrO₂, Pr₆O₁₁, Pr₂O₃) has been reported using Pr(thd)₃ (thd=2,2,6,6,-tetramethylheptane-3,5-dionate) (R. Lo Nigro, R. G. Toro, G. Malandrino, V. Raineri, I. L. Fragalà, Proceedings of EURO CVD 14, Apr. 27-May 2, 2003, Paris France (eds. M. D. Allendorf, F. Maury, F. Teyssandier), Electrochem. Soc. Proc. 2003, 2003-08, 915). However, the deposition temperature used (750° C.) is incompatible with the low deposition temperature generally required for microelectronics applications, where high growth temperatures can lead to problems such as increased dopant diffusion. The use of [Pr(thd)₃] may also lead to the presence in the Pr-oxide film of residual carbon, a common contaminant in oxide films grown using metal β-diketonates [Pr(hfa)₃(diglyme)] Pr(hfa)₃ diglyme (hfa=1,1,1,5,5,5-hexafluoro−2,4-pentanedionate, diglyme ═CH₃—O—(CH₂CH₂O)₂CH₃) was also investigated by these researchers, but led only to the unwanted oxyfluoride phase, PrOF.

Metal alkoxides have been widely used in the MOCVD of metal oxides, and generally allow lower growth temperatures than the more thermally stable metal β-diketonate precursors. There are no reports in the literature into the use of rare-earth alkoxide precursors in MOCVD. This is because the large ionic radius of the highly positively charged lanthanide(III) ions leads to the formation of bridging intermolecular metal-oxygen bonds, resulting in the majority of the simple alkoxide complexes being polymeric or oligomeric, with a corresponding low volatility which makes them unsuitable for MOCVD applications.

An object of this invention is to provide stable volatile rare earth metal oxide precursors suitable for use in chemical vapour deposition techniques.

It has been surprisingly found that the donor functionalised alkoxy ligand 1-methoxy-2-methyl-2-propanolate [OCMe₂CH₂OMe, mmp] is effective in inhibiting oligomerisation in praseodymium alkoxide complexes, as well as increasing the ambient stability of the complexes.

Accordingly the present invention provides rare earth metal precursors for use in MOCVD techniques having a ligand of the general formula OCR¹(R²)CH₂X, wherein R¹ is H or an alkyl group, R² is an optionally substituted alkyl group and X is selected from OR and NR₂, wherein R is an alkyl group or a substituted alkyl group.

Preferred precursors according to the invention have the following general formula:

M[OCR¹(R²)(CH₂)_(n)X]₃

wherein M is a rare earth metal, especially praseodymium, R¹ is H or an alkyl group, R² is an optionally substituted alkyl group and X is selected from OR and NR₂, wherein R is an alkyl group or a substituted alkyl group, n=1 to 4.

The preferred ligand of the formula OCR¹(R²)(CH₂)_(n)X (n=1) is 1-methoxy-2-methyl-2-propanolate (mmp) [OCMe₂CH₂OMe], but other donor functionalised alkoxide ligands may be used. These may include but are not limited to OCH(Me)CH₂OMe, OCEt₂CH₂OMe, OCH(Et)CH₂OMe, OC(Pr^(i))₂CH₂OMe, OCH(Pr^(i))CH₂OMe, OC(Bu^(t))₂CH₂OMe, OCH(Bu^(t))CH₂OMe, OCH(Bu^(t))CH₂OEt, OC(Bu^(t))₂CH₂OEt, OC(Pr^(i))₂CH₂OEt, OCH(Bu^(t))CH₂NEt₂, OC(Pr^(i))₂CH₂OC₂H₄OMe and OC(Bu^(t))(CH₂OPr^(i))₂.

The invention further provides a first method of making rare earth metal oxide precursors for use in MOCVD techniques comprising reacting HOCR¹(R²)(CH₂)_(n)X, wherein R¹, R² and X are as defined above, such as mmpH, with the corresponding rare earth metal alkylamide M(NR₂)₃ or silylamide precursor M(N(SiR₃)₂)₃, especially praseodymium silylamide precursor, Pr{N(SiMe₃)₂}₃, in appropriate molar proportions, wherein R=alkyl, such as, for example, Me, Et and Pr^(i).

According to the invention an alternative method of general synthesis of lanthanide and rare earth element complexes of the formula M[OCR¹(R²)CH₂X]₃ as defined above, such as, Ln(mmp)₃, involves the salt exchange reaction of Ln(NO₃)₃(tetraglyme) with appropriate molar equivalents of Na(M[OCR¹(R²)CH₂X]₃, such as Na(mmp), in tetrahydrofuran solvent. A similar method may be used for the preparation of Sc(mmp)₃ and Y(mmp)₃.

Precursors according to the invention may be used in depositing single or mixed oxide layers or films by conventional MOCVD, in which the precursor is contained in a metalorganic bubbler, or by liquid injection MOCVD, in which the precursor is dissolved in an appropriate inert organic solvent and then evaporated into the vapour phase using a heated evaporator. Appropriate solvents include aliphatic hydrocarbons, such as hexane, heptane and nonane, aromatic hydrocarbons such as toluene, and aliphatic and cyclic ethers. Additives such as polydentate ethers including diglyme, CH₃—O—(CH₂CH₂O)₂CH₃, triglyme, CH₃—O—(CH₂CH₂O)₃CH₃, tetraglyme, CH₃—O—(CH₂CH₂O)₄CH₃, and donor functionalised alcohols such as 1-methoxy-2-methyl-2-propanol HOCMe₂CH₂OMe (mmpH) may also be added to the solvent, as these may render the precursors of the invention, especially Ln(mmp)₃ (Ln=lanthanide such as La, Pr, Gd, Nd etc.), less reactive to air and moisture and may improve the evaporation characteristics of the precursor solution. The amount of additive added to the solvent will typically be in the region of 3 mol. equiv.: 1 mol. equiv. precursor. Lower amounts of additive are less effective but amounts of more than 3 mol. equiv. may be used.

The precursors may also be suitable for use in the deposition of praseodymium oxide films by other chemical vapour deposition techniques, such as atomic layer deposition (ALD).

The M[OCR¹(R²)(CH₂)_(n)X]₃ precursor may also be suitable for the deposition of rare-earth oxide films using non-vapour phase deposition techniques, such as sol-gel deposition and metal-organic decomposition, where the new complexes may undergo a more controlled hydrolysis reactions than simple M(OR)₃ complexes.

Other volatile rare earth precursors for use in MOCVD, ALD or sol-gel processes according to the invention may include lanthanide (rare-earth) elements, such as La, Ce, Gd, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb and Lu as well as Group IIIB elements including Sc and Y.

The precursors according to the invention can also be used, in combination with an appropriate silicon precursor for the MOCVD of lanthanide silicates, LnSi_(x)O_(y), and with appropriate co-precursors for the MOCVD of multi-component oxides, such as Pr_(x)M_(y)O_(z) containing praseodymium, or other rare earth metals with metals (M) from other groups of the periodic table.

The invention will now be further described by means of the following Examples and with reference to the accompanying drawings, in which:

FIG. 1 shows the X-ray crystal structure of [LiPr(mmp)₃Cl]₂;

FIG. 2 shows XRD spectra of Pr-oxide films deposited at 400° C. and 600° C. from [Pr(mmp)₃]. * denotes the dominant (101) reflection of the secondary θ-Pr₂O₃ phase;

FIG. 3 is an SEM image of a Pr-Oxide film deposited at 400° C. from [Pr(mmp)₃];

FIG. 4 is an X-ray diffraction pattern of a film of lanthanum oxide deposited at 450° C. from La(mmp)₃;

FIG. 5 is a scanning electron micrograph (SEM) of a fracture sample of the lanthanum oxide film of Example 4;

FIG. 6 is a ¹H NMR spectrum of a solution of La(mmp)₃ in toluene;

FIG. 7 is a ¹ NMR spectrum of a solution of Pr(mmp)₃ in toluene;

FIG. 8 is a ¹H NMR spectrum of a solution of La(mmp)₃ in toluene with 3 mol. equiv. of added tetraglyme;

FIG. 9 is a ¹H NMR spectrum of a solution of Pr(mmp)₃ in toluene with 3 mol. equiv. of added tetraglyme;

FIG. 10 shows ¹H NMR data of a solution of La(mmp)₃ in toluene with 3 mol. equiv. of added mmpH; and

FIG. 11 shows ¹H NMR data of a solution of Pr(mmp)₃ in toluene with 3 mol. equiv. of added mmpH.

EXAMPLE 1 Preparation of Pr(mmp)₃

MmpH (0.487 cm³, 4.23 mmol) was added to a solution of [Pr{N(SiMe₃)₂}₃] (0.878 g, 1.41 mmol) in toluene (80 cm³). The solution was stirred at room temperature for 10 min and then solvent and HN(SiMe₃)₂ was removed in vacuo to give a green oil.

Microanalysis: Found. C, 38.0; H, 6.60%. Calculated. For C₁₅H₃₃O₆Pr C, 40.01; H, 7.39%.

IR (ν cm⁻¹, neat liquid, NaCl plates): 2960 vs; 1496 in; 1458 s; 1383 m; 1357 s; 1274 s, 1229 vs, 1205 s; 1171 vs; 1113 vs; 1086 vs; 997 vs; 967 vs; 943 vs; 915 m; 828 w; 786 m; 730 s; 695 m.

NMR spectroscopy (CDCl₃; 400 MHz): (All resonances are broadened due to the paramagnetic Pr³⁺ (4f²). Integrals of these broad resonances are not reported due to the lack of precision): 100.5, 72.5, 69.7, 67.0, 64.0, 63.7, 62.4, 60.7, 58.4, 57.0, 56.0, 54.0, 53.5, 50.5, 48.2, 47.2, 42.2, 40.7, 19.1, 18.6, 18.0, 17.7, 15.3, 13.9, 12.7, 11.2, 3.1, 1.2, −4.7, −10.5, −11.8, −12.5, −13.0, −15.5, −19.0, −20.5, −24.4, −30.2, −40.1, −43.6, −45.3, −46.2, −54.0

The liquid nature of Pr(mmp)₃ precludes structural characterisation by single crystal X-ray diffraction, but In the presence of LiCl a crystalline complex with the formula [LiPr(mmp)₃Cl]₂ was isolated, providing further good evidence that the stoichiometry of the oil was [Pr(mmp)₃]. This complex was characterized by single crystal X-ray diffraction and its structure is shown in FIG. 1 of the drawings.

EXAMPLE 2

Pr(mmp)₃ was found to be a suitable precursor for the deposition of praseodymium oxide thin films by MOCVD. The praseodymium oxide films were deposited by liquid injection MOCVD using a 0.1M solution of Pr(mmp)₃ in toluene (14 cm³) to give a 0.1 M solution. The addition of tetraglyme CH₃—O—(CH₂CH₂O)₄CH₃ was found to stabilise the Pr(mmp)₃ solution by making it less reactive to air and moisture and improving the transport properties of the precursor. The growth conditions used to deposit Pr-oxide thin films by liquid injection MOCVD using a toluene solution of Pr(mmp)₃ are summarised in Table 1.

TABLE 1 Reactor pressure 1 mbar Evaporator temperature 170° C. Substrate temperature 350-600° C. Precursor solution concentration 0.1M in toluene with 3 mol equiv. of added tetraglyme Precursor solution injection rate 8 cm³ hr⁻¹ Argon flow rate 400 cm³ min⁻¹ Oxygen flow rate 100 cm³ min⁻¹ Substrates Si(100) Typical oxide growth rates 0.2 μm hr⁻¹ The identity of the films was confirmed as praseodymium oxide by as X-ray diffraction analysis (see FIG. 2 of the drawings), which indicated that the films comprise a major β-Pr₆O₁₁ phase with a minor component of the hexagonal θ-Pr₂O₃ phase. Reports elsewhere (R. Lo Nigro, R. G. Toro, G. Malandrino, V. Raineri, I. L. Fragalà, Proceedings of EURO CVD 14, Apr. 27-May 2, 2003, Paris France (eds. M. D. Allendorf, F. Maury, F. Teyssandier), Electrochem. Soc. Proc. 2003, 2003-08, 915) indicate that the proportions of the β-Pr₆O₁₁ phase and the θ-Pr₂O₃ can be controlled via the partial pressure of oxygen used during MOCVD growth.

Analysis of the films by scanning electron microscropy (SEM) showed that all the as-grown films exhibited smooth surfaces and uniform cross sectional thicknesses. The cross section of a film grown at 400° C. is shown in FIG. 3 of the drawings and shows no evidence of features such as columnar growth, which has been observed in other high-k dielectric films such as HfO₂ and ZrO₂.

Point energy dispersive X-ray analyses of the films indicates only Pr from the thin film and silicon from the underlying substrate material.

Further analysis by auger electron spectroscopy (AES) analysis of Pr-oxide films deposited from [Pr(mmp)₃] (see Table 2) showed that the films are pure Pr-oxide, with no detectable carbon.

TABLE 2 Deposition temperature Composition (at. %) Pr/O Sample (° C.) Pr O C ratio 1 350 30.3 69.7 ND 2.3 4 600 32.9 67.1 ND 2.0

EXAMPLE 3 Preparation of La(mmp)₃

MmpH (3 mol. equiv.) was added to a solution of [La{N(SiMe₃)₂}₃] (1 mol equiv.) in toluene. The solution was stirred at room temperature for 10 min and then solvent and HN(SiMe₃)₂ was removed in vacuo to give the product.

Microanalysis: Found. C, 40.0; H, 7.4%. Calcd. For C₁₅H₃₃O₆La C, 40.2, H, 7.4%.

IR (ν cm⁻¹, neat liquid, NaCl): 2960 vs; 1496 m; 1457 s; 1384 in; 1357 s; 1261 s; 1229 vs; 1172 vs; 1090 vs; 1084 vs; 1001 s; 965 vs; 944 s; 914 m; 841 m; 821 m; 794 s; 730 s; 695 m.

NMR spectroscopy C₆D₆ (400 MHz) Main resonances: δ (ppm): 3.16 br singlet; 3.08 br singlet (total 5H); 2.65 singlet; 1.27 singlet (6H). Other resonances 3.2-4 ppm, complex pattern (total approx 2H); 1.2-1.8 ppm, complex pattern (total approx 4H).

The same general preparative method can be used for the synthesis of other M(mmp)₃ complexes where M=Group IIIB element such as Sc and Y, or a lanthanide (rare earth) element such as, Ce, Gd or Nd.

EXAMPLE 4

La(mmp)₃ was found to be a suitable precursor for the deposition of lanthanum oxide thin films by MOCVD. Growth conditions used to deposit La-oxide thin films by liquid injection MOCVD using a toluene solution of La(mmp)₃ are summarised in Table 3.

TABLE 3 Substrate Temperature 300-600° C. Evaporator Temperature 170° C. Pressure 1 mbar Injection Rate 8 cm³h⁻¹ Solvent Toluene + 3 mol. eq. tetraglyme Concentration 0.1M Argon flow rate 400 cm³min⁻¹ Oxygen flow rate 100 cm³min⁻¹ Run time 1 h

The X-ray diffraction pattern (see FIG. 4 of the drawings) of a film deposited at 450° C. exhibits three dominant diffraction peaks attributed to the (100), (002) and (101) reflections measured at 2θ values of 25.1°, 27.9° and 29.7° respectively. The approximate ratio of intensities of these peaks is consistent with the random powder diffraction pattern of La₂O₃ with a hexagonal structure. The width of the observed reflections is notable and consistent with either very small grain size or the transformation of the oxide to the monoclinic LaO(OH) arising from exposure of the film to the ambient environment.

The atomic composition of the LaO_(x) films was determined using Auger electron spectroscopy (AES), and the results are summarized in Table 4.

TABLE 4 AES analysis of La-oxide films grown by MOCVD Argon flow Deposition rate Oxygen Com- Film temperature (cm³ flow rate position (atom %) no. (° C.) min⁻¹) (cm³ min⁻¹) La O O/La 317 300 400 100 29.0 71.0 2.4 314 350 400 100 35.0 65.0 1.8 318 400 400 100 33.8 66.2 1.9 309 450 400 100 31.3 68.7 2.2 316 500 400 100 33.0 67.0 2.0 313 550 400 100 33.7 66.3 2.0 315 600 400 100 31.8 68.2 2.1 319 450 500 0 34.4 65.6 1.9 320 450 250 250 32.3 67.7 2.1

The O:La ratios of 1.8-2.4 are consistent with the films being La₂O₃ containing excess oxygen (expected O:La ratio in La₂O₃=1.5). Carbon was not detected in any of the films at the estimated detection limit of <0.5 at.-% and carbon-free La-oxide films were obtained, even in the absence of oxygen, so that [La(mmp)₃] is effectively acting as a “single-source” oxide precursor.

A scanning electron micrograph (SEM) of a fracture sample from that lanthanum oxide film deposited at 450° C. is shown in FIG. 5 of the drawings. A columnar growth habit is discernable which has associated ‘hillock’ features on the free growth surface causing a fine surface roughening effect.

EXAMPLE 5 Preparation of Nd(mmp)₃

MmpH (3 mol. equiv.) was added to a solution of [Nd{N(SiMe₃)₂}₃] (1 mol. equiv.) in toluene. The solution was stirred at room temperature for 10 min. and then solvent and HN(SiMe₃)₂ was removed in vacuo to give the product.

Microanalysis: Found: C, 38.8; H, 6.9%. Calcd. For C₁₅H₃₃O₆Nd, C, 39.7; H, 7.33%.

Infrared data: recorded as thin film between NaCl plates (cm⁻¹) 2963 vs; 1496 m; 1457 s; 1384 m; 1357 s; 1275 s; 1231 vs; 1173 vs; 1117 vs; 1086 vs; 1010 s; 968 vs; 915 m; 823 m; 793 a; 730 s; 695 m

¹H NMR (CDCl₃) [resonances are broadened due to paramagnetism of Nd³⁺ (4f³)]: 35.1, 31.7, 30.9, 18.8, 17.4, 15.8, 12.6, 11.5, 8.2, 5.6, 1.2, −9.0, −9.6, −18.2, −24.5, −25.6, −26.0, −55.8, −57.5

EXAMPLE 6 Use of Additives to Stabilise Precursor Solutions

The ¹H NMR spectra of [La(mmp)₃] and [Pr(mmp)₃] in toluene solution are shown in FIGS. 6 and 7, respectively. The complexity of the ¹H NMR. data indicates that the structure of both these compounds are extremely complex, and particularly in the case of La, the complexity of the spectrum increases with time. This indicates that there is a significant amount of irreversible molecular aggregation in solution. This process is probably due to condensation reactions to form oxo-bridged oligomers; such reactions are well documented in lanthanide alkoxide chemistry. The resonances are also broadened, possibly due to inter-molecular ligand exchange reactions, commonly observed in solutions of metal alkoxide complexes.

Significantly, the addition of 3 mol. equivalents of the polydentate oxygen donor ligand tetraglyme, (CH₃—O—(CH₂CH₂O)₄CH₃), to the precursor solutions results in much simpler ¹H NMR spectra (FIGS. 8 and 9). This strongly suggests that the presence of (CH₃—O—(CH₂CH₂O)₄CH₃) inhibits molecular aggregation. The observation that the tetraglyme resonances are not subject to paramagnetic shifting in [Pr(mmp)₃][tetraglyme] indicates that the tetraglyme is not bonded directly to Pr, and we, therefore, conclude that stable adducts of the type [Ln(mmp)₃(tetraglyme)] are not formed.

The addition of one mole excess of [mmpH] (HOCMe₂CH₂OMe) to toluene solutions of La(mmp₃) or Pr(mmp)₃ also results in simpler ¹H NMR spectra (see FIGS. 10 and 11 of the drawings) and has a similar stabilizing effect. The simplicity of the ¹H NMR spectra indicates that mmp and mmpH are in rapid exchange and there is no uncoordinated mmpH. The addition of tetraglyme or mmpH to solutions of [Ln(mmp)₃] was found to enhance air/moisture stability as well as prevent aggregate formation. The mechanism of this stabilization has not been established, but it is likely to be due to some form of shielding of the lanthanide metal centre from oxygen atoms on mmp ligands on neighbouring molecules.

EXAMPLE 7 Preparation of Gd(mmp)₃

[Gd(mmp)₃] was synthesised by the addition of mmpH (3 mol. equiv.) to a solution of [Gd{N(SiMe₃)₂}₃] (1 mol equiv.) in toluene. The solution was stirred at room temperature for 10 min and then the solvent and liberated HN(SiMe₃)₂ were removed in vacuo to give the product as a green oil. The product was confirmed by elemental microanalysis for C and H.

EXAMPLE 8 Growth of Gadolinium Oxide Using Gd(mmp)₃

Gadolinium oxide films were deposited on Si(100) substrates at 1 mbar using a liquid injection MOCVD reactor. The films were deposited over the temperature range 300-600° C. using a 0.1M solution of [Gd(mmp)₃] in toluene, with 3 equivalents of added tetraglyme using the same growth conditions to those given in Table 3. Gadolinium oxide films were also grown on GaAs(100) using a 0.1M solution of [Gd(mmp)₃] in toluene, with 3 equivalents of added tetraglyme, in the absence of added oxygen.

The films grown on Si(100) and GaAs(100) substrates were confirmed to be Gadolinium oxide by Auger electron spectroscopy (AES) as shown in the following Table 5:—

TABLE 5 Atomic composition (at.-%) of gadolinium oxide films measured by AES* Depo- Argon Oxygen sition flow flow rate Film temp. rate (cm³ (cm³ no. Substrate (° C.) min⁻¹) min⁻¹) Gd O O/Gd 1 Si(100) 450 500 0 37.6 62.4 1.7 2 GaAs(100) 450 500 0 36.9 63.1 1.7 *H not analysed for.

X-ray diffraction data for Gd₂O₃ films showed that at growth temperatures above 450ÙC , the GdO_(x) films crystallize as Gd₂O₃ with a C-type structure exhibiting a preferred (111) orientation. At lower growth temperatures the data exhibited no diffraction features suggesting an amorphous disordered structure.

The diffraction pattern of the Gd₂O₃ film deposited on GaAs(100) at 450ÙC was dominated by the (222) reflection. This indicates a strong preferred orientation or a heteroepitaxial relation with the underlying GaAs.

EXAMPLE 9 Stabilisation of M(mmp)₃ (M=Rare Earth Element) Precursor Solutions by the Addition of Donor Additives

The ¹H NMR spectra of [La(mmp)₃] and [Pr(mmp)₃] in toluene solution are shown in FIGS. 6 and 7, respectively. The addition of 3 mole equivalents of the polydentate oxygen donor ligand tetraglyme, —(CH₃—O—(CH₂CH₂O)₄CH₃) to the precursor solutions of M(mmp)₃ (M=La, Pr) results in much simpler ¹H NMR spectra (FIGS. 8 and 9 of the drawings) and renders the precursor solutions less air sensitive, and significantly improves the evaporation characteristics of the precursor solution in liquid injection MOCVD applications.

This strongly suggests that the presence of (CH₃—O—(CH₂CH₂O)₄CH₃) inhibits molecular aggregation. The observation that the tetraglyme resonances are not subject to paramagnetic shifting in [Pr(mmp)₃][tetraglyme] indicates that the tetraglyme is not bonded directly to Pr, and we, therefore, conclude that stable adducts of the type [Ln(mmp)₃(tetraglyme)] are not formed.

The addition of one mole excess of 1-methoxy-2-methyl-2-propanol, [HOCMe₂CH₂OMe] (mmpH) to solutions of M(mmp)₃ (M=rare earth element) in toluene has a similar stabilizing effect (see FIGS. 10 and 11). In the case of [Ln(mmp)₃(mmpH)] the simplicity of the ¹H NMR spectrum indicates that mmp and mmpH are in rapid exchange and there is no uncoordinated mmpH. The addition of tetraglyme or mmpH to solutions of [Ln(mmp)₃] was found to enhance air/moisture stability as well as prevent aggregate formation.

The mechanism of this stabilization has not been established, but it is likely to be due to some form of shielding of the Ln metal centre from oxygen atoms on mmp ligands on neighbouring molecules.

EXAMPLE 10 Growth of Neodymium Oxide Using Nd(mmp)₃

Neodymium oxide films were deposited on Si(100) substrates at 1 mbar using a liquid injection MOCVD reactor. The films were deposited over the temperature range 250-600° C. using a 0.1 M solution of [Nd(mmp)₃] in toluene, with 3 equivalents of added tetraglyme employing the equivalent growth conditions to those given in Table 3. Neodymium oxide films were also grown on GaAs(100) using a 0.1M solution of [Gd(mmp)₃] in toluene, with 3 equivalents of added tetraglyme, in the absence of added oxygen.

The films grown on Si(100) and GaAs(100) substrates were confirmed to be neodymium oxide, Nd₂O₃, by Auger electron spectroscopy (AES) as shown in the following Table 6:—

TABLE 6 Atomic composition (at.-%) of the NdO_(x) films measured by AES* Argon Oxygen flow flow Deposition rate rate Film temperature (cm³) (cm³ no. Substrate (° C.) min⁻¹) min⁻¹) Nd O O/Nd 1 Si(100) 300 400 100 37 63 1.7 2 Si(100) 450 400 100 40.1 59.9 1.5 3 Si(100) 500 400 100 38.7 61.3 1.6 4 Si(100) 450 500 0 41.2 58.8 1.4 5 Si(100) 450 450 50 41.8 58.2 1.4 6 Si(100) 450 350 150 41.7 58.3 1.4 7 Si(100) 450 300 200 45.5 54.5 1.2 8 Si(100) 450 250 250 42.1 57.9 1.4 9 GaAs(100) 450 500 0 40.6 59.4 1.5 *H not analysed for 

1. Rare earth metal precursors, for use in MOCVD techniques having a ligand of the general formula OCR¹(R²)CH₂X, wherein R¹ is H or an alkyl group, R² is an optionally substituted alkyl group and X is selected from OR and NR₂, wherein R is an alkyl group or a substituted alkyl group.
 2. A precursor as claimed in claim 1 having the following general formula: M[OCR¹(R²)(CH₂)_(n)X]₃ wherein M is a rare earth metal R¹ is H or an alkyl group, R² is an optionally substituted alkyl group and X is selected from OR and NR₂, wherein R is an alkyl group or a substituted alkyl group, n=1 to
 4. 3. A precursor as claimed in claim 1 or 2, wherein M is praseodymium.
 4. A precursor as claimed in any one of claims 1 to 3, wherein the ligand of the formula OCR¹(R²)(CH₂)_(n)X is 1-methoxy-2-methyl-2-propanolate (mmp) [OCMe₂CH₂OMe].
 5. A precursor as claimed in claim 1 or 2, wherein the ligand is chosen from the group of OCH(Me)CH₂OMe, OCEt₂CH₂OMe, OCH(Et)CH₂OMe, OC(Pr^(i))₂CH₂OMe, OCH(Pr^(i))CH₂OMe, OC(Bu^(t))₂CH₂OMe, OCH(Bu^(t))CH₂OMe, OCH(Bu^(t))CH₂OEt, OC(Bu^(t))₂CH₂OEt, OC(Pr^(i))₂CH₂OEt, OCH(Bu^(t))CH₂NEt₂, OC(Pr^(i))₂CH₂OC₂H₄OMe and OC(Bu^(t))(CH₂OPr^(i))₂.
 6. Pr(mmp)₃.
 7. La(mmp)₃.
 8. Gd(mmp)₃.
 9. Nd(mmp)₃.
 10. A method of making rare earth metal oxide precursors for use in MOCVD techniques comprising reacting a ligand HOCR¹(R²)(CH₂)_(n)X, wherein R¹ is H or an alkyl group, R² is an optionally substituted alkyl group and X is selected from OR and NR₂, wherein R is an alkyl group or a substituted alkyl group, n=1 to 4, with the corresponding rare earth metal alkylamide M(NR₂)₃ or silylamide precursor M(N(SiR₃)₂)₃ precursor, Pr{N(SiMe₃)₂}₃, wherein R=alkyl, in appropriate molar proportions.
 11. A method as claimed in claim 10, wherein the ligand is mmpH.
 12. A method as claimed in claim 10 or 11, wherein the rare earth metal compound is praseodymium. silylamide.
 13. A method as claimed in claim 10 or 11, wherein in the rare earth metal alkylamide or silylamide R is selected from Me, Et and Pr^(i).
 14. A method of making lanthanide and rare earth element complexes of the formula M[OCR¹(R²)CH₂X]₃, wherein R¹ is H or an alkyl group, R² is an optionally substituted alkyl group and X is selected from OR and NR₂, wherein R is an alkyl group or a substituted alkyl group and n=1 to 4, comprising the salt exchange reaction of M(NO₃)₃(tetraglyme) with an akali metal or alkaline earth metal salt of [OCR¹(R²)CH₂X]₃ (mmp) in tetrahydrofuran solvent.
 15. A method as claimed in claim 14, wherein the alkali metal salt is Na(mmp).
 16. A method as claimed in claim 14 or 15, wherein the rare earth metal complex prepared is selected from Sc(mmp)₃, Y(mmp)₃ and Ln(mmp)_(a).
 17. A method of depositing single or mixed oxide layers or films by MOCVD using a rare earth metal precursors having a ligand of the general formula OCR'(R²)CH₂X, wherein R¹ is H or an alkyl group, R² is an optionally substituted alkyl group and X is selected from OR and NR₂, wherein R is an alkyl group or a substituted alkyl group, in which the precursor is contained in a metalorganic bubbler.
 18. A method of depositing single or mixed oxide layers or films by MOCVD using a rare earth metal precursors having a ligand of the general formula OCR¹(R²)CH₂X, wherein R¹ is H or an alkyl group, R² is an optionally substituted alkyl group and X is selected from OR and NR₂, wherein R is an alkyl group or a substituted alkyl group, or by liquid injection MOCVD, in which the precursor is dissolved in an appropriate inert organic solvent and then evaporated into the vapour phase using a heated evaporator.
 19. A method as claimed in claim 18, wherein the solvent is selected from aliphatic hydrocarbons, aromatic hydrocarbons and aliphatic and cyclic ethers.
 20. A method as claimed in claim 19, wherein the aliphatic hydrocarbon solvent is selected from hexane, heptane and nonane.
 21. A method as claimed in claim 19, wherein the aromatic hydrocarbon solvent is toluene.
 22. A method as claimed in any one of claims 18 to 21, wherein the solvent contains an additive selected from polydentate ethers and donor functionalised alcohols.
 23. A method as claimed in claim 22, wherein the polydentate ether is selected from diglyme, CH₃—O—(CH₂CH₂O)₂CH₃, triglyme, CH₃—O—(CH₂CH₂O)₃CH₃, and tetraglyme, CH₃—O—(CH₂CH₂O)₄CH₃.
 24. A method as claimed in claim 22, wherein the donor functionalised alcohol is 1-methoxy-2-methyl-2-propanol HOCMe₂CH₂OMe (mmpH).
 25. A method as claimed in claim 22, 23 or 24, wherein the amount of additive added to the solvent is at least 3 mol. equiv.:mol. equiv of precursor.
 26. A method as claimed in claim 25, wherein the amount of additive added to the solvent is in the region of 3 mol. equiv.
 27. A method as claimed in any one of claims 18 to 26, wherein the precursor has the following general formula: M[OCR¹(R²)(CH₂)_(n)X]₃ wherein M is a rare earth metal R¹ is H or an alkyl group, R² is an optionally substituted alkyl group and X is selected from OR and NR₂, wherein R is an alkyl group or a substituted alkyl group, n=1 to
 4. 28. A method as claimed in any one of claims 18 to 27, wherein M is praseodymium.
 29. A method as claimed in any one of claims 18 to 28, wherein the ligand of the formula OCR¹ (R²)(CH₂)_(n)X is 1-methoxy-2-methyl-2-propanolate (mmp) [OCMe₂CH₂OMe].
 30. A method as claimed in any one of claims 18 to 28, wherein the ligand is chosen from the group of OCH(Me)CH₂OMe, OCEt₂CH₂OMe, OCH(Et)CH₂OMe, OC(Pr^(i))₂CH₂OMe, OCH(Pr^(i))CH₂OMe, OC(Bu^(t))₂CH₂OMe, OCH(Bu^(t))CH₂OMe, OCH(Bu^(t))CH₂OEt, OC(Bu^(t))₂CH₂OEt, OC(Pr^(i))₂CH₂OEt, OCH(Bu^(t))CH₂NEt₂, OC(Pr^(i))₂CH₂OC₂H₄OMe and OC(Bu^(t))(CH₂OPr^(i))₂.
 31. A method as claimed in any one of claims 18 to 30, wherein the precursor is Pr(mmp)₃.
 32. A method as claimed in any one of claims 18 to 30, wherein the precursor is La(mmp)₃.
 33. A method as claimed in any one of claims 18 to 30, wherein the precursor is Gd(mmp)₃.
 34. A method as claimed in any one of claims 18 to 30, wherein the precursor is Nd(mmp)₃.
 35. A method of depositing single or mixed oxide layers or films by atomic layer deposition (ALD) using a rare earth metal precursors having a ligand of the general formula OCR¹(R²)CH₂X, wherein R¹ is H or an alkyl group, R² is an optionally substituted alkyl group and X is selected from OR and NR₂, wherein R is an alkyl group or a substituted alkyl group.
 36. A method as claimed in claim 35, wherein metal oxide film being deposited is a praseodymium oxide film.
 37. A method of depositing single or mixed oxide layers or films by a non-vapour phase deposition technique using a rare earth metal precursors having a ligand of the general formula OCR¹(R²)CH₂X, wherein R² is H or an alkyl group, R² is an optionally substituted alkyl group and X is selected from OR and NR₂, wherein R is an alkyl group or a substituted alkyl group.
 38. A method as claimed in claim 37, wherein the technique used is selected from sol-gel deposition and metal-organic decomposition.
 39. A method as claimed in any one of claims 17 to 38, wherein M is selected from La, Ce, Gd, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tin, Yb and Lu as well as Group IIIB elements including Sc and Y.
 40. A method as claimed in any one of claims 17 to 39 carried out with appropriate co-precursors, for the MOCVD of multi-component oxides. 